The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for magnetic resonance elastography (“MRE”).
Non-alcoholic fatty liver disease (“NAFLD”) is a spectrum of liver diseases that represent the most common cause of chronic liver disease in the United States, most likely affecting up to thirty percent of the adult U.S. population. NAFLD can exist as simple steatosis, steatosis with inflammation, and non-alcoholic steatohepatitis (“NASH”). Of these three conditions, NASH involves actual hepatocyte injury and has the potential to progress to cirrhosis in 10-20 percent of cases. Because NAFLD is strongly associated with obesity, there is also concern that the number of cases of NAFLD, and therefore NASH, will increase given the epidemic of obesity in the United States. Currently, the gold standard for definitively distinguishing NASH from other forms of liver disease is by way of a liver biopsy: an invasive procedure not without risk. However, there are a number of new non-invasive imaging modalities that show promise in replacing liver biopsy in the screening of patients with NAFLD.
One such non-invasive method is magnetic resonance elastography (“MRE”), which is a dynamic elasticity imaging technique that mimics the centuries-old diagnostic technique of palpation by applying harmonic mechanical waves to tissue and quantitatively assessing the stiffness of tissue by analyzing the pattern of wave propagation using MRI. In effect, MRE provides a technique for “palpation by imaging.” That is to say, MRE allows for “palpation” of deep tissues like the liver and benefits from the fact that the elastic moduli of human tissues vary over a wide range, allowing for enhanced contrast in MRE imaging. Prior research in measuring tissue elasticity involved the application of static tissue stress with measurement by either ultrasound or MRI saturation imaging. MRE is different in that it uses propagating waves, rather than static stress, as a probe.
Conventional harmonic MRE makes use of an external motion driver, a special MRI imaging sequence with oscillating motion-sensitizing gradients, and a data inversion algorithm. The external drivers are usually either an electromechanical driver, a piezoelectric stack driver, or an acoustic speaker pneumatic system that create multiple periods of sinusoidal motion at a single frequency in the tissue of interest. The imaging sequence typically uses several periods of mechanical motion and several phase offsets to sample the wave field. A reconstruction algorithm can then be used to quantify the mechanical properties of the tissue. Clinical studies have established harmonic MRE as a possible alternative to biopsy for assessing liver fibrosis.
Techniques for studying the propagation of transient impulses as it relates to material stiffness have been around for many years in various fields, including acoustics, optics, and geophysics. Transient wave analysis has also been attempted in connection with MRE, as described by P. McCracken, et al., in “Mechanical Transient-Based Magnetic Resonance Elastography,” Magnetic Resonance in Medicine, 2005; 53:628-639. In this method, phase-contrast MRE acquisition was employed, but an external driver was used as the motion source. In addition, this method was implemented for MRE of the brain, not the liver.
A significant amount of work has been done over the years to measure motion in the brain due to intrinsic pulsations of the cerebrospinal fluid (“CSF”) and vasculature due to cardiac motion. In recent work, phase-contrast MR imaging of the brain has provided researchers with images of transient wave propagation in the brain that they have been able to use to provide tissue stiffness information, as described by S. Zhao, et al., in “Auto-Elastography of the Brain,” Proceedings of the ISMRM, 2009; 713; and by A. J. Pattison, et al., in “Poroelastic MRE Reconstructions of Brain Acquired with Intrinsic Activation,” Proceedings of the ISMRM, 2010; 3404. These methods employed a gradient-echo, phase-contrast, flow imaging sequence. Such a method, while useful for brain imaging, would be inadequate for imaging in the liver where motion from the heart would confound imaging in addition to supplying transient wave propagation useful for MRE.
Recent work has used intrinsic motion from the heart to measure tissue strain within the liver, as described by S. Chung, et al., in “Liver Stiffness Assessment by Tagged MRI of Cardiac-Induced Liver Motion,” Magnetic Resonance in Medicine, 2011; 65(4):949-955. This method uses images of tagged MR magnitude signal to measure tissue displacement, rather than using phase-contrast imaging techniques. Additionally, in this method, motion of the wave produced by the heart is not tracked, and tissue stiffness is not calculated. Instead, the method measures strain, which is a relative, not quantitative, measure of tissue mechanical properties. A similar MR tagging technique has also been used for measuring liver strain due to respiratory motion, as described by H. Watanabe, et al., in “MR Elastography of the Liver at 3T with Cine-Tagging and Bending Energy Analysis: Preliminary Results,” European Radiology, 2010; 20(10):2381-2389. In this method, cardiac-induced motion was not measured and, once again, no transient wave propagation analysis was performed to measure stiffness.
Ultrasonic imaging techniques have been used since the 1980s to track liver motion due to cardiac pulsations, though not always to measure tissue stiffness. Ultrasound-based techniques, while faster than MR techniques, are limited to imaging only through acoustic windows of the body, such as through intercostal spaces, and to measuring only one component of motion.
As noted above, hepatic MRE currently requires the use of an external vibration source (“driver” or “actuator”) that produces motion outside of the body that propagates into the body and into the liver. The MRI system can then image this wave propagation in the liver and report the tissue stiffness. The required use of an external driver can be limiting because the driver is extra equipment that must be attached to the patient, can be a source of discomfort for some patients, and has decreased efficiency of getting motion into the liver in obese patients. Being able to perform MRE without the use of an external driver would offer certain advantages over the existing methods.
In light of the foregoing methods, it would be advantageous to provide a method for magnetic resonance elastography that does not rely on an external driver to produce motion in the liver and that can rapidly acquire images so as to not be negatively impacted by cardiac motion.